Vitreous enamel coatings



United States Patent 3,159,33 Vl'fitEGllS ENAMEL Cs'BA'IiNQS Francis 23. Gibson, Lansing, llh, and Louis lv-icDar-ri l,

Hammond, Ind, designers to inland (Ihicago, IEL, a corporation of Delaware No Filed June 1, 195?, Ser. No. 82 3330 5 Claims. (1. 29-627) This invention relates to vitreous enamel coatings. It is more specifically concerned with the production of vitreous enamel coatings by firing a ground'coated metal base in a non-oxidizingatmosphere.

Because enamels form an excellent protective coating for metals they are extensively used for effecting a protection of the coated article or surface from ambient atmospheric conditions which may be deleterious to the metal base. The vitreous enamel coating consists of a layer of glass which is thermally fused to a suitable metal base such as steel. In the enamcling process a so-called ground coat consisting of a slip of the pulverized enamel constituents is applied to a metal base and the ground-coated base is fired in an enameling furnace at an elevated temperature sutlicient to effect the fusing of the ground coat.

in order to effect the adherence of the ground coating to the base metal, adherence agents are included in the ground coat formulation. In general, oxides of nickel, cobalt, manganese, and others are used as well as salts of these metals which under the enameling conditions will form oxides. It has been found in the enameling industry that cobalt oxides or cobalt salts convertible to the oxide under enameling conditions are especially effective adherence agents.

In conventional enameling operations it is necessary to thoroughly clean the metal base prior to applying the enamel. The cleaning, in the case of sheet metal, generally consists of degreasing, pickling in acid, rinsing in a neutralizing bath, sometimes followed by a nickel solution dip to improve enameling behavior during firing, after which the articles are washed and dried. Cast iron articles are cleaned by grinding and sand blasting. In all instances, however, the surface of the base metal to be enameled is given a bright surface.

After the application of the slip, the ground coat is fired at temperatures as high as l500-1600 F. In conventional enameling operations the firing or burning of the enamel coating onto the base metal is carried out in a furnace in which the atmosphere surrounding the material being enameled is not controlled. Accordingly, the firing is normally carried out in an oxidizing atmosphere.

Under certain conditions it is sometimes desirable to carry out the firing operation in a neutral atmosphere wherein the atmosphere is substantially neutral or even a reducing atmosphere. It has been found that if the conventional enameling operation in which a vitreous enarne coating is to be applied to a base metal surface which has been thoroughly cleaned to produce a bright surface and conventional cobalt-containing ground coats are utilized, the vitreous enamel coating will not adherently fuse to the base metal surface but will produce a defective coating which will peel off upon normal handling of the enameled base metal. In an illustrative enameling process wherein it is desirable to employ a non-oxidizing atmosphere in the firing step, conventional embossed, steel, floor plates are treated to vitreous enamel coat only the embossed areas. Conventional enameling techniques employing a pro-cleaning step cannot be used to produce the desired result because under these conditions the uncoated surfaces of the base metal bubble and inordinately scale under the firing conditions. If the floor plate is not pro-cleaned,

Elbhfil Fatented Feb. 16, 1955 a satisfactory coating cannot be produced and excess oxidation of the exposed uncoated surfaces occurs. In order to avoid the formation of the surface bubbles as well as the scaling, it is necessary to carry out the firing operation in a controlled atmosphere furnace employing a non-oxidizing atmosphere whereby the uncoated surfaces of the metal base are not deleteriously affected by the furnace atmosphere. Under these conditions, however, it has been found thatthe vitreous enamel coating will not adhere to the metal base if the base has been cleaned in the conventional manner.

According to this invention it has been found that the vitreous enamel coating of suitable metal bases, employing conventional ground coats which are fired in a nonoxidizing atmosphere, can be satisfactorily effected by forming on the surface of the base metal an adherent tight, light blue oxide coating or the like prior to applying the ground coat. By initially providing a suitable blue oxide coating, the subsequent enameling process wherein the ground coat is fused to the base metal can be carried out in a controlled, non-oxidizing atmosphere furnace to provide a vitreous enamel coating which firmly adheres to the metal base surface and permits any uncoated metal base surfaces to be subjected to the firing without being deleteriously affected by the formation of surface bubbles or inordinate scaling.

The adherent tight, light blue oxide coating employed is thinly and uniformly produced on at least the areas which are to be subsequently vitreous enamel coated.

Thereafter a suitable ground coat slip, preferably containing cobalt oxide or a cobalt salt convertible'to the oxide under firing conditions, is applied to the areas ofthe metal base surface which are to be enameled. The ground coated metal base is thenfired in a non-oxidizing furnace I atmosphere for a time and at a temperature sufiicient to fuse the enamel and develop a fused bond between the enamel and the base metal. Thereafter the coated base metal is cooled for storage purposes.

The blue oxide coating which is formed on the metal base is an iron oxide which has a rich blue color inherently produced by a suitable oxidizing process. 'When the metal base employed is steel or iron the coating can be formed on the iron or steel surface by heating the base metal in an open furnace to a temperature within the transformation range and then cooling in air. For example, a suitable blue oxide coating is produced on low carbon steel by heating it to above 1000" F. in on oxidizing atmosphere or coolingit from above 1900 F. in an oxidizing atmosphere. 1

It has been found that various metals including iron 7 under proper conditions will form adherent coatings in hot oxidizing environments. It has been found that a tight or adherent scale suitable for, use in the instant invention is obtained when the specific-volume of the oxide is equal to or greater than the specific volume of the metal. in the case of iron the ratio of Specific volume of iron oxide Specific volume of iron ferrous base metal can be heated in a furnace to about V 1350 F. while inaint I 1 ing composition:

'0 0.1'2 -.40. .f; ,v l a oxygen-free tirr ng atmosphere, it has special application rolling was carried outatf atempe'ratur'eof about 160i)" F. Thereafter the rolled 'plate was cooled singly or in stacksto' room temperature after the laststand in'air..

5 in'the furnace. After cooling the ferrous base'metal to Editing a reducingatmosphere- ,withjsnsasroa temperature within. the range of 900120Q F., the

ferrous metal base is discharged into the open air to produce'a desirable tight,.light blue oxide on the surface. Steam can also be used to form the desired blue 7 oxide coating by annealing a'ferrous metal base at a 1 this invention will be termed, a tight, light blue oxide coating. Although in general 'in-carryingoutthe instant 1 T invention. a ferrous base .me'tal will be uti'lized, it is -.evident that other base metals which have been surface .treated to produce the desired tight, light blue oxide tal fromcoatingfthereon can 'be employed. ln these instances,

'- forexample, a-suitable base metal such as copper or aluminum can be'surface'treated by dipping 'the'sheet or base metal article into a solution of a suitable iron 'sa'lt such asferric or ferrous sulfate or chloride to wet the entire surface thereof and thereafter subjecting the coating of a suitable ferrous deposit to anoxidizing technique wherein an adherent tight, light blue iron oxide is laid 'downonthe base metal surface.

Although in most'instances itwill be necessary to ing, in certain steel shaping processes, such as the forming' Off the aforementioned embossed floor plate, car 'ried out. at elevated temperatures, the conditions of time and temperature are-such :as to inherently produce on :the steel-surfaces the'required "tight, lightiblue oxide coating Without'the need for additional treating processes.

For example, a ferrous metal ingot having the'follow- .19 max.

Under these' conditions there was I I 7 formed the desired tight, light blue oxide coating.

=In ground coating the metal base conventional enamel light blue oxide coating'is then coatedfwith the ground coat by spraying, brushing, roller coating, or im process the metal base witha suitable surface trea -7 :ment in order to form the tight, lightblue oxide coatenamel ingredients. A-typicalfrit will have the following composition. 1

The frit is then mixedwith the following constituents in aballfimill to provide the water suspension of the enamel ingredients:

" I e Percent Clay 7 Borax 0.25 Magnesium carbonate 0.125 Sodium nitrite 0.125 0.125

The enamel constituents are milled to a fineness of about 37% retain on a 200 mesh American Standard screen.

The metal base uponwhich has been formed the tight,

' F: for a suitable time, generally about 4-10 minutes, to

i was heated and 'rolledto produce embossed floor plate. 4

fuse the enamel and the metal base and provide the vitreous enamel coating. In carrying out the firing phase ofthe enameling'process ofthis invention, a controlled'firing atmosphere substantially free from oxygen and preferably a neutral atmosphere is used.

7 Although the instant invention can be employed gen= erally for the formationof vitreous enamel coatings on suitable metal bases which require the use of a controlled,

jin' the formation of the e'narnelcoatings in spaced areas on a selectedmetal'base wherein the remaining areas of the coatedjmetal'base are not covered by the vitreous fenamel.

Although there a're anumber of decorative" and functional installations where this type of vitreou's enamel .coatin'g'can be employeditsuse is especially illustrated compositions 'can be used, although cobalt oxide ground coatings are preferred. In general the ground coat contains a 'suitable frit capableiof forming low melting glasses. The frit contains the glass-formingconstituents,

V of the ernbossed floor plate. The instant invention pro' iluxes, oxidizingaQgntS, as wellgasj adherence agents. A

' I variety of hit compositions can 'be used depending upon the properties of the finished article; i 'In general, however, the'frit comprises an admixture of quartz and feldspartogether with fluxes such as borax, fluorspar, cryolyte, soda-lash, sodium nitrite, and litharge to'which are added adherence agents such as c'obalt'oxi'dealone .or'in coma The frit bina'tion-with nickeland manganese oxides.

. abrasive particles to the -surfaceof. thernetal base; Un}

V is carried out'without further treatment'of the plate with the exceptionlofa possible degre asing step. 111 pp y is then mixed with opaeifiers such as theoxides of tin,

z'irconiur'rnor antimony, together with clay which also pending medium. Otherfsuspending agents such as borax' j contributes to the opacity; but also functions .as a susand magnesium "carbonate can also "be employed. Q The' solid constituents are" admixed with Water in "a ball mill to provide a suitable watersuspe'nsion of the pulverized ftionalfi'oor plate. Steel shapes of this form are generally rolledernploying suitable roll structures which depress a substantial port'ion'of the plate-surface leaving raised or "embossed areas. Although such plates are generally rolled in-this manner to provide a non-skid surface, itis frequently desirable to enhance the non-skid effectiveness vides an ideal wayfor applying abrasive particles to the embossed .areasjby utilizing a vitreous enamel ground coat as the bonding agent for applying and retaining the der the rolling .c nditiOnsI employed in forming the; floor plate a tight, light blue oxide is inherently produced. iccor'dingly theyitreous enameling processof this invention the abrasive particles; aconventional Water .suw ig j enamel ,constituents'is applied onlyto thev emb 111335 of the floor plate by spraying, or brushir g th 'meial f s i l ground coat utilizing a s i' fih in. i

order to' avoid covering the depressed areas: of t metal 7 plate. A preferred method f bating-the'ernbossed areas utilizes'a roller in-whi'chtorgl tlie embossed areasare confi iclsd iviththe roller ,a's; ilt applies the water SuSPemiQn of the enamel constituents to the base metal. In this manner only about the upper three fourths of the embossment is coated with the ground coating A suitable abrasive such as sand, carborundum, or other types of granular or particulate abrasives are spread over the floor plate by a suitable distributing hopper, preferably while the ground coating is still moist and tacky. Thereafter the excess sand which does not adhere to the moist slip on the embossments or is laying on the uncoated areas of the floor plate is removed and the plate dried in a suitable oven in order to dry the moist slip. The pre ared floor plate is then fired in a neutral atmosphere furnace for a time and at a temperature sufiicient to fuse the enamel to the embossments and develop a bond between the abrasive particl s and the metal base. By utilizing this technique the exposed or uncoated areas of the floor plate are not deleteriously afiected by the treating process and there is provided a floor plate having the embossments coated with an abrasive material which is firmly held thereon and provides a surface with enhanced anti-skid characteristics.

To illustrate the effectiveness of the instant invention, two identical pieces of as-rolled A" mild steel floor plate were obtained. One piece was subjected to a pickling operation in order to completely remove all of the oxide scale and provide a sample in accordance with conventional pre-enameling cleaning techniques. The other sample was not so cleaned but was Water washed to remove any grease or oil from the surface. Both samples were then coated with a water suspension of enamel constituents by means of a short-napped cloth roll to coat the upper /4 of the embossments. Abrasive particles con sisting of finely divided sand (less than 50 mesh) were spread evenly over the hour plates by a suitable hopper and the excess sand removed by a suitable vacuum type cleaner. Both samples were dried and thereafter fired in a neutral atmosphere furnace at a temperature within the range of about 1500-1600 F. for 7 minutes. After cooling, the prepared floor plates were then subiected to a drop-Weight impact test of 5 pounds dropped 20 inches. The fioor plate was placed so that the weight struck directly on top of a vitreous enamel-coated embossment. The plate prepared in accordance with conventional enameling techniques wherein the plate was pro-cleaned prior to applying the ground coating provided a door plate which manifested almost a complete lack of enamel adherence as evidenced by the complete removal of the enamel from the embossrnent after the impact of the dropped weight. On the other hand the enamel coating produced on the floor plate having a tight, light blue oxide contained thereon had excellent adherence and only the enamel in theimmediate vicinity of the blow was shattered by the dropped weight.

It is therefore apparent from the foregoing discussion of this invention that, in the application of vitreous enamel coatings to suitable base metals wherein it is necessary to utilize a firing furnace having a controlled atmosphere which is substantially free from free oxygen, the formation of a tight, light blue oxide coating on the metal surface is essential for the required adherence of the vitreous enamel formed thereon. The instant invention has special application wherein it is desirable to proenamel coating on metal sheets or scattered enameled areas on fiat sheets by means of a suitable stencil which only coats the desired areas of the base metal. it is also apparent that a finish coat of vitreous enamel can be applied using the same or diiierent slips than the ground coat when color combinations, light colors, additional protection, etc. are required. It is obvious that there will be various other modifications which can be utilized employing the teachings of this invention without departing from the scope thereof. Such variations in the invcntion claimed by the appended claims will be apparent to those sldlled in the art.

What is claimed is:

LA method for producing a non-skid metal floor plate which comprises hot-rolling a mild steel floor plate having a surface thereof raised into scattered bosses, forming during hot rolling a tight light blue oxide coating on said bosses, roll coating only a major portion of the area of said bosses with an enamel slip without cleaning after hot rolling, spreading abrasive particles on the coated scattered bosses, and thereafter firing the floor plate at an 2. A method as recited in claim 1 wherein said oxiderorming step comprises forming an oxide having a specific volume equal to 1.77 times the specific volume of said mild steel floor plate.

3. A method for producing a floor plate with scattered enamel areas thereon, said method comprising:

hot-rolling a mild steel floor plate having a surface of raised scattered bosses;

forming a tight, light blue oxide coating on said bosses;

roll-coating only a major portion of the area of said bosses with an enamel slip applied atop said light, light blue oxide;

and thereafter firing the floor plate at an elevated temperature in a controlled, oxygen-free atmosphere for a time sufiicient to form a vitreous enamel coating fused to said bosses.

4. A method as recited in claim 3 wherein: 1

said tight, light blue oxide is formed during said hotrolling; i

said method further comprising retaining said oxide formed during hot-rolling, and roll-coating said enamel slip atop said retained oxide.

5. A method as recited in claim 3 and comprising spreading abrasive particles on the enamel coated bosses before said firing.

References Cited in the file ofthis patent UNITED STATES PATENTS OTHER REFERENCES The Making, Shaping and Treating of Steel, US. Steel Corp, 7th ed., 1957, page 625. 

3. A METHOD FOR PRODUCING A FLOOR PLATE WITH SCATTERED ENAMEL AREAS THEREON, SAID METHOD COMPRISING: HOT-ROLLING A MILD STEEL FLOOR PLATE HAVING A SURFACE OF RAISED SCATTERED BOSSES; FORMING A TIGHT, LIGHT BLUE OXIDE COATING ON SAID BOSSES; ROLL-COATING ONLY A MAJOR PORTION OF THE AREA OF SAID BOSSES WITH AN ENAMEL SLIP APPLIED ATOP SAID TIGHT, LIGHT BLUE OXIDE; AND THEREAFTER FIRING THE FLOOR PLATE AT AN ELEVATED TEMPERATURE IN A CONTROLLED, OXYGEN-FREE ATMOSPHERE FOR A TIME SUFFICIENT TO FORM A VITREOUS ENAMEL COATING FUSED TO SAID BOSSES. 